Conventionally, an electrolyte that is formed by impregnating a non-aqueous electrolytic solution into a film having pores in which the film is referred to as a separator has been generally used as an electrolyte of a lithium secondary battery. In recent years, a lithium secondary battery (such as a polymer battery) using a polymer electrolyte made of a polymer other than such a liquid-phase electrolyte has attracted attention.
The polymer battery uses an electrolyte in gel form in which a liquid electrolytic solution has been impregnated in the polymer. Since the liquid electrolytic solution is retained in the polymer, it is difficult for the liquid electrolytic solution to leak out. Therefore, the polymer battery has the advantages that the safety of the battery is enhanced, and also the contour of the cell may be formed freely.
The polymer electrolyte has the low conductivity of the lithium ions when being compared with the electrolyte composed of only the liquid electrolytic solution. Thus, due to the low conductivity of the lithium ions, a method for reducing the thickness of the polymer electrolyte is being used. However, when the polymer electrolyte is reduced to be thin in this way, the mechanical strength of the polymer electrolyte is reduced, the positive electrode and the negative electrode are short-circuited at the time of manufacture of the battery, to thus cause a problem of easily destroying the polymer electrolyte.
Korean Patent Application Publication No. 10-2006-1743 proposed a lithium secondary battery comprising a positive electrode, a negative electrode, and an electrolyte to allow reversible intercalation/de-intercalation of lithium, in which the electrolyte comprises a non-aqueous organic solvent containing a lactone compound having a cyclic carbonate and an alkyl substituent, a lithium salt and a gel-forming compound.
However, since the secondary battery has a structure that the positive electrode and the negative electrode are separated by a gel-type electrolyte, the ionic conductivity of the electrolyte falls in the case of forming a thick film as the electrolyte, and a short circuit occurs between the positive electrode and the negative electrode in the case of forming a thin film as the electrolyte.
Korean Patent Application Publication No. 10-2004-84117 proposed a method of manufacturing a lithium ion polymer battery in which a gel polymer electrolyte is prepared by comprising: stacking a positive electrode, a separator, and a negative electrode in sequence, to thereby being inserted into an aluminum laminate film; injecting a precursor that is formed of a mixture of a liquid electrolyte, a polymerized polymer, a reactive monomer or a macromonomer, a polymerization initiator, and the others, to then perform vacuum sealing; and performing polymerization by maintaining for up to one hour and 30 minutes in a constant temperature chamber of 60° C. ˜80° C.
The method of manufacturing the lithium ion polymer battery is characterized in that IPN (Interpenetrating Polymer Network) or HDDA (hexanedioldiacrylate), and a reactive modifier are added in the precursor to thus change a composition ratio thereof to thereby change the physical properties of the gel polymer electrolyte, in which the IPN or HDDA is formed by using one or more acrylate monomers that can react with a polymer having a polymerized acrylate radicle, and the reactive modifier is formed by using at least one acrylate having two or more reaction radicles such as triethyleneglycoldimehtacrylate or tetraethyleneglycoldiacrylate.
The method of manufacturing the lithium ion polymer battery employs a nonwoven fabric separator made of PE (polyethylene) or PP (polypropylene), to thus allow a low porosity and a thick coating layer, to thereby deteriorate ion conductivity.